Abstract

Despite inconclusive findings over the role of stretching modalities on functioning and injury risk (Lauersen et al., 2014, British Journal of Sports Medicine 48, 871-877) developing flexibility is still considered an essential part of sports conditioning and rehabilitation. Modified Proprioceptive neuromuscular facilitation (mPNF) is a stretching modality used extensively with athletic populations to develop flexibility (Hindle et al., 2012, Journal of Human Kinetics, 31, 105-113). Importantly, the intensity of the voluntary isometric contraction component of mPNF prescription lacks empirical consensus, leading to variable mPNF practices (Westwater-Wood et al., 2010, Physical Therapy Reviews, 15, 23-27). The purpose of this study was to investigate the most effective percentage of maximum voluntary isometric contraction in improving hamstring flexibility using active knee extension and active single leg raise. This study used a quasi-experimental crossover design. With institutional ethical approval, twenty two symptomatic (single leg raise <700) healthy college athletes (mean age 18.09 years, s = 1.51; males = 17, females = 5) were recruited. Following testing familiarisation, participants received an acute treatment of contract-relax mPNF requiring a 20%, 50%, or 70% maximum voluntary isometric contraction (MVIC) component. Desired %MVIC was established using a subjective BORG-CR scale with verbal anchors (c.f. Day et al., 2004, Journal of Strength and Conditioning Research, 18, 353-358). Additional to the contract-relax mPNF, a sham treatment (passive static stretch) was used. The treatment sequence was randomised using a Latin square. Protocol for each treatment was conducted by a graduate sports therapist. Pre- and post-treatment active knee extension and active single leg raise was recorded using a high speed video camera, and analysed using Dartfish TeamPro software. Effect size calculations revealed medium pre- and post- treatment effect size at 20% (0.58) and 70% (0.56) MVIC , small at 50% MVIC (0.41), and trivial (0.035) for the sham treatment across both measures. Analysis of magnitude based inference revealed 20% and 70% MVIC was equally beneficial (>98%) for active knee extension and active single leg raise compared to 50% MVIC (80.9 and 87.5%) and the sham treatment (34.4 and 20%). Of interest, the sham treatment was found to be 18.7% harmful on ASLR measurement. Results suggest that whilst mPNF is an effective hamstring stretching modality, sub maximal %MVIC are comparable to maximal intensities in improving active knee extension and single leg raise after an acute treatment of contract-relax mPNF. Practitioners should be aware of this finding to reduce the risk of iatrogenic issues.